In an electrophotographic printer or a copying machine, an image corresponding to image data is formed on a photosensitive member by using a laser scanning system. The optical resolution of a laser scanner depends on the point spread function of the laser beam impinging on an image forming surface.
This point spread function is similar to a Gaussian distribution. The energy distribution in the image forming surface is determined by the convolutional integral of the point spread function and the input signal function. To obtain a contrast of 100 percent in the image forming surface, the beam diameter of the laser beam in the image forming surface must be 0. As this would entail the point spread function here being a delta function, it is physically impossible.
FIG. 10 shows the spatial frequency characteristics of the optical contrast of a laser scanner in the conventional art, and it illustrates an example of the optical contrast in the image forming surface. In this figure, the l/e.sup.2 beam diameter of the laser in the image forming surface is 80 .mu.m, the x-axis represents the spatial frequency (lp/mm: line-pairs per mm), and the y-axis represents the optical contrast in the image forming surface.
As shown by the continuous line 10-1, the optical contrast in the image forming surface when the spatial frequency is 8 lp/mm, which means the corresponding optical resolution is 16 dot/mm, is 0.5 approximately.
If the required contrast in the image forming surface at the threshold frequency is 80 percent and the point spread function in the image forming surface of the static optical system is a perfect Gaussian distribution, the required beam diameter is about 40 .mu.m.
If the required contrast is 90 percent, the beam diameter must be 30 .mu.m or less, as the higher the required contrast is, the smaller the beam diameter is required to be. If the beam diameter is required to be too small, it is not implementable in practice.
In the conventional art, the laser scanner also has a relatively high contrast in the spatial frequency range where it is not required, viz. above 8 lp/mm in FIG. 10, and the optical system also provides unnecessary contrast for high frequency noise included for example in the drive signal to an acoustic optical modulator. With this type of optical system it is not possible in principle to obtain the ideal contrast characteristics, shown by the broken line 10-2 in FIG. 10.
Japanese unexamined patent publication Sho 55-36893 discloses prior art related to this kind of scanner. In the scanner disclosed in this publication, an acoustic optical modulator is used as a modulating means supplied with image data, and an image focusing lens is positioned between the acoustic optical modulator and a rotating polygonal mirror, M is the lateral magnification of the image focusing lens, V.sub.1 is the scanning velocity and V.sub.s is the acoustic wave propagation velocity inside the acoustic optical modulator. The contrast is improved by making M=-V.sub.1 /V.sub.s.
As described above, it is impossible to obtain a good frequency response in the conventional art, because of the limits in principle to the contrast of the image formed in the image forming surface, and the frequency characteristics are inevitably similar to those shown by the continuous line 10-1 in FIG. 10.
Although the scanner disclosed in the above described publication has good high-frequency characteristics by exploiting the coherence of the scanning beam, as the image focusing lens is positioned before the rotating polygonal mirror, there is a large fluctuation in the light quantity of the laser beam in the image forming surface and in the frequency response of the laser beam. Thus, to obtain stable characteristics, the facets of the rotating polygonal mirror must be large.